WIM sensors of this kind are embedded in roads to cross at least one wheel track but preferably to cover the entire lane destined for road vehicles. Also known are longer WIM sensors of up to 4 m in length. The WIM sensors are able to determine the dynamic wheel loads of wheels passing over them to finally derive therefrom the total dynamic load of the vehicle, optionally along with a trailer, that acts onto the WIM sensor or WIM sensors as it passes over the sensor(s).
WIM sensors of various designs based on different measurement principles are available. Of importance for the present invention are WIM sensors of the type as described above.
A first example of a WIM sensor of the aforementioned type is described in U.S. Pat. No. 5,265,481A. Two piezoelectric measuring elements are arranged one above the other in the interior space of the hollow profile. Between them is an electrode that connects all these measuring element pairs and is itself electrically connected to a plug or an output wire at the end of the hollow profile. Support elements are disposed on both sides, i.e., above and below the measuring elements, to ensure that the measuring elements are positioned level between the pressing surfaces within the interior space. The pressing surfaces are connected to one another on both sides via the profile edge and ensure uniform pretensioning of the measuring elements by keeping the profile edge under tension.
For an exact measurement it is important that the amount of pretension is the same over the entire length of the WIM sensor. For this purpose, it is first of all required that the hollow profile is manufactured uniformly over its entire length as regards the wall thickness of the profile edges, distance of the pressing surfaces from each other and evenness of the pressing surfaces. Furthermore, also the entire sensor package that is inserted into the hollow profile must show the same thicknesses over all measuring elements. This is important not only for the measuring elements themselves but in particular for the support elements and the electrode. In U.S. Pat. No. 5,265,481A each measuring element pair has its own pair of support elements. It is relatively difficult to assemble this type of arrangement because all components of the sensor package must be precisely aligned and must be kept from moving with respect to each other until the sensor package has been fully inserted into the clamped hollow profile.
An additional difficulty arises from the use of steel electrodes that have been rolled to comply with the allowed thickness. As a result of rolling the strip temporarily assumes a bent form that winds along the entire length of the strip. Even though the tolerances for thickness and width are fulfilled the lateral offsets may well vary by more than 2 mm per meter. This results in too many rejects since only those sections of the strip can be used that are straight because otherwise lateral short circuits will occur.
A WIM sensor is described in WO2013104080A1 that follows the same measurement principle. The object of this document is to produce the sensor package in a more cost effective way while complying with the required tolerances. This is achieved by long holding elements connected by form-locking connections at their end faces. Thus, on the one hand holding elements can be used that are considerably shorter than the total length of the WIM sensor and on the other hand the entire sensor package is easily integrated in the hollow profile due to the form-locking connections. It is significantly easier to achieve the dimensional accuracy for shorter holding elements than for very long ones. In contrast to the one known from U.S. Pat. No. 5,265,481A that includes pairs of measuring elements, this sensor package has only one measuring element at each measuring point, and thus the electrode connecting all measuring elements is covered with an insulation.
Finally, a WIM sensor of the type describe herein above is known from EP0654654A1. It comprises an insulating plate on which are supported two different types of measuring elements arranged side by side or one above the other. The insulating plate is partially provided with conductor paths.
Advantageously, the insulation and the conductive layer of the electrode are integrated in one component because in this case they are no longer able to move with respect to each other.